Letter to the Editor

Stellar X-ray heating of planet atmospheres

¹ INAF - Osservatorio Astronomico di Cagliari, Strada 54, Loc. Poggio dei Pini, 09012 Capoterra (CA), Italy e-mail: ccp@ca.astro.it
² INAF - Osservatorio Astronomico di Palermo, Piazza Parlamento 1, 90134 Palermo, Italy e-mail: ciarave@astropa.unipa.it, giusi@astropa.unipa.it

Received: 24 July 2006
Accepted: 28 August 2006

Abstract

Aims.To investigate the effects of the stellar X-ray irradiation on planet atmospheres, we study the X-ray transfer and energy deposition in a hydrogen rich gas.

Methods.We construct an accurate X-ray transfer model taking both photoionization and Compton scattering into account; the electron energy deposition is followed by tracking the discrete exchange processes between electrons and the gas mixture.

Results.Exospheric heating rates are derived as functions of the pressure in model atmospheres using a wide range of X-ray luminosity, spectral hardness representative of different stellar ages, and distances from the parent star. The computed heating rates suggest that X-ray irradiation might be an important heating source in planetary exospheres even at large distances from the parent star.